How do you decrease the amount of Vertices in an object?
I doubt there is a way to restore the original mesh (with the exact topology) after applying subsurf modifier. What you can do is use an Un-Subdivide option of the Decimate Modifier (expect some loss in detail though). General rule is if your applied subsurf level was set to 2 you set the unsubdivide level to 4 etc. In my case I set the Un-Subdivide level to 10 on a model with the level 5 subsurf applied.


Note: After appling Decimate modifier you may go to Edit Mode, select all the vertices (A) and hit Alt+J to get rid of some tris to make the topology even more similar to the original mesh.